1. Field
The presently disclosed embodiments relate generally to the control of light emitting devices such as Cold Cathode Fluorescent Lamps and Light Emitting Diodes. More specifically, the disclosed embodiments relate to controlling the backlighting of Liquid Crystal Displays.
2. Background
Cold Cathode Fluorescent Lamps (CCFLs) are now commonly used for backlighting Liquid Crystal Displays (LCDs) in notebook and laptop computer monitors, car navigation displays, point of sale terminals and medical equipment. The CCFL has quickly been adopted for use as the backlight in notebook computers, and various portable electronic devices because it provides superior illumination and cost efficiency. These applications generally require uniformity of display brightness and illumination intensity.
Typically, liquid crystal material, separated from a CCFL backlighting device by a diffuser layer, polarizes the light for each display pixel. A high voltage DC/AC inverter is required to drive the CCFL because this lamp uses a high Alternating Current (AC) operating voltage. With the increasing size of the LCD panel, multiple lamps are required to provide the necessary illumination. Therefore, an effective inverter is required to drive multiple CCFL arrays.
Intensity of illumination is determined by the operating current applied to the CCFL by an inverter. In conventional multiple lamp panel arrays, either each lamp must be driven by its own costly inverter, or one shared inverter sets the operating current of all the lamps to a current determined by a preset amount of total current for all the lamps.
However, each lamp varies in brightness and intensity due to age, replacement and inherent manufacturing variations. Applying the same reference current to each lamp, without adjusting for individual lamp variations, creates a different intensity of illumination for each lamp. Varying illumination intensities causes visible undiffused lines to be displayed. Conventional single inverter circuits cannot individually sense and adjust the operating current for each lamp in order to equalize the illumination intensity across multiple lamp array display panels.
As the market place has driven down the cost of CCFLs, resulting in widespread use of multiple lamp array display panels, the demand for inverter quality, economy and functionality has increased. Conventional types of backlights for LCD devices are not fully satisfactory in illumination intensity uniformity. Thus, there is a need in the art for an economical inverter capable of individually sensing and adjusting the current applied to an array of CCFLs in multiple lamp LCD displays.